ABSTRACT The overall goal of this proposal is to better understand how the brain is affected in Chronic Mountain Sickness (CMS) or Monge's disease. The brain is one of the major targets of CMS, as manifested by the frequently seen CNS symptoms such as headache, dizziness, sleep disturbance, and mental confusion. MRI studies have revealed cerebral edema and multiple ischemic foci in the brain of CMS patients, suggesting that brain cell damage/death occurs in CMS which may lead to the above neurological symptoms. Therefore, understanding the vulnerability of brain cells to hypoxia in CMS is critical for developing the therapeutic strategy of CMS. Recently, we obtained skin biopsies from CMS patients and non-CMS healthy highlanders and re-programmed the skin cells into induced pluripotent stem cells (iPSCs). The iPSCs were then differentiated into neuroprogenitor cells and further into astrocytes and neurons. We then examined the acid- base regulatory mechanisms and the tolerance/vulnerability to hypoxia/ischemia stress in these astrocytes/neurons. Our preliminary data show that astrocytes derived from CMS exhibited increased acid extrusion rate, and CMS astrocytes/neurons were more vulnerable to hypoxic/ischemic challenges than non- CMS cells. Therefore, we hypothesize that astrocytes/neurons in CMS patients have altered acid-base regulatory mechanisms which render neurons/astrocytes more vulnerable to hypoxia. We will test this hypothesis via two specific aims: 1) examine the expression and activities of NHE1 and NBCe1 in CMS and non-CMS astrocytes/neurons under normoxia and hypoxia conditions; 2) investigate the role of NHE1 and NBCe1 in the cell death of CMS astrocytes/neurons following hypoxia. We will use H+ and Na+ imaging system together with the genetic knockdown and pharmacological inhibition of NHE1 and NBCe1 to determine their role in the tolerance/vulnerability of CMS astrocytes/neurons to hypoxic stress. A novel feature of our study is the establishment of an in vitro CMS model that was directly derived from CMS and non-CMS highlanders. It is highly likely from our studies that we will make a number of discoveries that will help us design better CMS therapeutic strategies.